During gasification of carbonaceous material such as e.g. black liquor a combustible gas is formed, hereafter referred to as process gas, comprising carbon monoxide, carbon dioxide, and hydrogen. The process gas obtained varies in purity according to the quality of the original fuel with regard to sulphur content and ash composition.
A number of gasification processes operate at temperatures so high that inorganic compounds form molten ashes, while other processes operate at lower temperatures where the ashes remain in solid or partly solid form during the course of gasification. Gasification of black liquor is normally performed at temperatures between 700.degree. and 1000.degree. C.
SE-B-448,173 discloses a process for the recovery of chemicals and energy from cellulose spent liquors obtained from the pulping industry and shows partial combustion of black liquor at reactor pressures above ambient, whereby a melt of the mineral part of the spent liquor is obtained together with a process gas, which is cooled by passaging thereof through a liquid trap containing an alkaline solution while the mineral melt is dissolved in said alkaline solution. The process gas then passes through different washing stages one of which may use a sodium hydroxide solution as the washing liquid.
A characteristic feature of black liquor is its high reactivity which, i.a., can be related to the high oxygen content of the dry solids, and the catalytic effect of sodium and sodium compounds.
Another characteristic feature is that an inorganic phase having a low melting point is developed during gasification of black liquor. This melt phase consists essentially of water soluble sodium and sulphur compounds in contrast to conventional coal ashes which contain only a small amount of water soluble compounds.
It is of importance that the carbon conversion during gasification is kept at a high level and therefore the temperature in the gasification reactor should be kept above 700.degree. C.
During combustion of kraft spent liquors a fine aerosol containing sodium compounds is often formed. This aerosol is very difficult to separate mechanically, and this in combination with melt droplets carry over represents a serious problem in connection with gasification of black liquor.
The sulphur in the black liquor forms gaseous, liquid and solid compounds during gasification. In the melt phase the sulphur is present mainly in the form of sodium sulphide and in the gas phase as hydrogen sulphide.
The chemical equilibriums of importance in the present washing system are as follows: EQU Na.sub.2 CO.sub.3 +H.sub.2 O.fwdarw.NaHCO.sub.3 +NaOH (1) EQU Na.sub.2 S+H.sub.2 O.fwdarw.NaHS+NaOH (2) EQU 2NaOH+CO.sub.2 .fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O (3) EQU 2NaHS+CO.sub.2 .fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 S (4) EQU NaOH+H.sub.2 S.fwdarw.NaHS+H.sub.2 O (5)
During absorption of hydrogen sulphide in sodium alkaline solutions Na.sub.2 S and NaHS are formed. At high CO.sub.2 partial pressure and high concentration of sulphide ions in the liquid bulk hydrogen sulphide can be released during formation of sodium carbonate. This is undesirable and prevented in the present invention as shown in the description below.
Selective absorption of hydrogen sulphide in the system is encouraged by short contact time and a large contact surface between the wash liquid and the gas bulk. This is best achieved by atomizing the wash liquid e.g., in one or more spray nozzles. The absorption process can be carried out in a spray tower with or without packing.
The object of the present invention is to promote capture of hydrogen sulphide according to equilibriums (4), (5) and to suppress sodium carbonate formation according to equilibriums (1), (3) and (4). The used wash liquid in the first stage contains a high concentration of hydrogen sulphide ions and should therefore not be used as wash liquid in the subsequent washing stages of the present invention, as this would encourage reformation of undesirable gaseous hydrogen sulphide. To avoid undesirable reformation of gaseous hydrogen sulphide and to promote the capture thereof the wash liquids in the later washing stages should contain a considerably higher concentration of hydroxide ions relative to hydrogen sulphide ions.
Environmental protection is becoming increasingly important for the process industry and no less important for the pulping industry where i.a., closed chemical loops are desirable. Alkaline liquids internally generated in the kraft mill are suitable for use as wash liquids in the present invention and the spent wash liquors are suitable for preparation of cooking liquor.